In the case of conventional insulating blocks, for example perforated bricks, gas-concrete blocks and blocks including cement-bound lightweight building materials, the attempt is made to optimize the heat-insulating capacity by using as lightweight a building material as possible. Consequently, use is made of high-porosity clays for bricks, foamed concrete, pumice, pearlite or the like. However, this method is restricted by virtue of the limited resistance to compression of the lightweight building materials.
The prior art further improves the heat-insulating capacity of a given block by a skilled arrangement of air slots which pass through completely, or at least to a major extent, from one side of the block to the other and transversely with respect to the heat-flow direction. In particular, the heat-insulating capacity is improved by slot-shaped cavities which are aligned in the longitudinal direction of the block and are offset with respect to one another transversely with respect to the heat-flow direction. However, the elongate cavities which are produced in bricks by the extrusion process and thus pass through the bricks weaken the stability, in particular the resistance to transverse tension, of the insulating block. Consequently, it is not possible to go below a minimum cross-sectional surface area of heat-conducting webs in the heat-flow direction.
It is known that, with a predetermined thickness of the longitudinal webs running transversely with respect to the heat-flow direction, the optimum average slot width or the average number of slots following one after the other in the heat-flow direction can be calculated (Swiss Patent Specification 476 181, 482 882 and 516 057). The average slot width is understood as being the cross-sectional surface area of a usually elongate cavity divided by its greatest extent transverse to the heat-flow direction. The number of slots is averaged over a multiplicity of cuts through the brick which are guided in the heat-flow direction, and corresponds to a more conventional parameter, namely the number of slot rows. The cavity cross-sections are usually of shapes elongated transverse to the heat-flow direction, for example ellipses, rectangles, trapeziums, cuboids, triangles, etc. The cavities may also be square, round or of shapes with five, six and more sides.
In the case of blocks consisting of fired clay, web thicknesses of 6 mm and more are conventional. If the web thickness is reduced, for example to 4 or 2 mm, then, following on from the abovementioned patent specifications, the optimum number of slots increases in an extremely pronounced manner, with the result that it is no longer possible to produce bricks with the theoretically determined optimum number of slot rows since overly high pressures occur during the extrusion of the clay compositions. For example, for a brick of a thickness of 30 cm, with the web thickness being 2 mm according to Leitner (see abovementioned CH-PS 516 057) or Amrein (see abovementioned CH-PS 476 181), the slot width would have to be 3.5 mm. Consequently, over 50 rows of slots would be necessary in order approximately to reach the theoretically determined maximum. Bricks of a thickness of 30 cm which are produced today usually have 17 rows of slots, and not more than 21 rows of slots. 30 rows of slots would, at this moment in time, constitute a significant limit to producibility.
A further possibility for producing heat-insulating blocks consists in producing the block with a plurality of larger cavities and, in order to restrict the heat loss in the cavities, filling said cavities subsequently with insulating inserts consisting of extremely different materials, this, however, constituting an operation involving a high degree of outlay.
Conventional insulating blocks which have been optimized with the above methods achieve coefficients of thermal conductivity of 0.12 W/mK or worse, at best 0.15 W/mK in the case of bricks.